4Water Quality Impact Assessment

4.1Introduction

4.1.1.1This section presents the Water Quality Impact
Assessment (WQIA) for the construction and operational phases of the Project.

4.1.1.2The aim of the WQIA is to assess and evaluate
impacts of the proposed Project upon water sensitive receivers within the Study
Area and to identify measures to avoid or otherwise reduce predicted impacts to
within acceptable levels.

4.2Objectives

4.2.1.1This section has been compiled in accordance
with the evaluation criteria and assessment guidelines as presented in Annexes
6 and 14 respectively of the EIA-TM, and with reference to the requirements of
Clause 3.4.1 of the EIA Study Brief.

4.2.1.2Key objectives of the water quality impact
assessment include the following:

·To collect
and review background information on the existing and planned water system(s)
and their respective sensitive receivers;

·To characterise water and sediment quality and water sensitive receivers
based on existing information or appropriate site survey and tests;

·To identify and analyse physical, chemical and biological disruptions of
marine water system(s) arising from the project construction and operation;

·To predict, quantify and assess any water quality impacts arising from
the Project on the water system(s) and the sensitive receivers by appropriate
mathematical modelling techniques;

·To identify and evaluate the best practicable dredging methods to
minimize dredging and dumping requirements;

·To evaluate the potential of and associated water quality impacts
arising from accidental vessel collisions within the Project area during
construction and maintenance of the wind farm;

4.3.2.1Defines the boundaries of the ten local Water
Control Zones (WCZs) and specifies the requirements Water Quality Objectives
(WQOs).The WQOs set limits for
different parameters to be achieved for maintaining the water quality within the
WCZs.In accordance with the Study
Brief, the Study Area of the project should cover the MirsBay,
Port Shelter, Eastern Buffer and Junk Bay WCZs.Table 4.1
summarises the WQOs for these WCZs.

4.3.3.1Technical Memorandum on Environmental Impact
Assessment Process (EIAO-TM), Annexes 6 and 14specifies
the assessment method and criteria for water quality impact assessment.This section follows the details of the
assessment criteria and guidelines for evaluating water pollution.

4.3.4Water Supplies Department Water Quality Objectives

4.3.4.1Stipulate a set of water quality objectives for
water quality at seawater intakes.Table 4.2 presents the relevant criteria.The suspended solids and dissolved
oxygen requirements are most relevant to this EIA study.

4.3.5Technical Memorandum on Standards
for Effluents Discharged into Drainage and Sewerage Systems Inland and Coastal
Water

4.3.5.1Provides guidance on the permissible effluent discharges
for foul sewers, storm water drains, inland and coastal waters.Should any effluent be generated from
this Project, the effluent quality should comply with the standards for
effluents discharged into the inshore waters or marine waters of Junk Bay WCZ,
Eastern Buffer WCZ and Mirs Bay WCZ.

4.3.6.1Sets
out the procedure for seeking approval to dredge/ excavate sediment and the
management framework for marine disposal of dredged/ excavated sediment. The
Technical Circular also specifies the requirements for determination of
sediment quality, classification of sediment and disposal arrangement for the
sediment.

4.4.1.1Various
foundation options and construction methods have been evaluated in order to
minimize the potential environmental impacts of the proposed Project.Details of alternative site and
construction options are presented in Section 2 of this EIA Report.Table 4.3
summarizes the preferred foundation and substructure options for Project
development.

4.4.1.2Foundation
installation requires the removal of water from inside of the suction caissons
to the ambient water through pumping.The pumped out water may contain a certain amount of sediment.Transmission power cables and collection
power cables will be installed by jetting, with the exception of the section
located within JunkBay.This section, approximately 3-km long,
requires anchor protection which will require the deployment of a dredger.Both jetting and dredging would cause
release of sediment into the water body.

4.4.2.1A fine grid model has been developed using the
Delft3D suite of models for prediction of the impacts due to sediment
dispersion in the construction phase and the changes in hydrodynamic regime
within the Study Area after the completion the Project.Details of the model setup and
calibration are presented in the Report
on Wind Farm Model Calibration (Appendix 4A
refers).

4.4.2.2The Delft3D-PART module using a particle
tracking method has also been used to simulate the concentration distribution
of suspended solids (SS).Depletion
of dissolved oxygen (DO) is calculated based on the modelled SS concentrations
at WSRs. The concentrations of the other pollutants at the WSRs are estimated
based on the results predicted by the model.

4.4.3.1The proposed works will lead to the release of
sediment and contaminants into the ambient water, resulting in potential water
quality impacts.Tidal currents are
the controlling factor for the dispersion of sediment disturbed by foundation
installation and cabling works.Sediment release rates for different activities are estimated below
based on the characteristics of the preferred construction methods and
equipment:

4.4.3.2Rock amour protection involving the dredging of
a trapezoidal trench is proposed for the cable in JunkBay.The approximate volume of dredged
sediment is 135,000m3, based on an approximately 3km long trapezoidal trench of nominal 3m depth (shown in Figure 3.2). The sediment
would be extracted by closed-grab dredging followed by backfilling with
rock.The maximum dredging
rate for the grab dredger is not expected to exceed 6,300m3 per day.
Sediment loss rates depend on the
size of the grab dredger, small dredgers generally have higher sediment loss
rates.The approximate range of the
sediment loss rates for large and small dredgers is between 12 kg/m3
and 25 kg/m3 (John et al.,
2000).In this study, a
conservative value of the sediment loss rate for the grab dredger of 25 kg/m3
is assumed.

4.4.3.3A portion of dredging work in JunkBay
is carried out near to the shore and may not be allowed between 19:00 and 07:00
hours on normal weekdays.It is assumed
that the work would be carried out over 12 hours per day with 6 working days
per week for the transmission cable section in JunkBay.The worst-case scenario for dredging
includes two grab dredgers operating at the same time with a minimum separation
of 100 m.Estimation of the
sediment release rate for grab dredging is given below:

Grab size = 11 m3

Working hours = 12 hr/day

No. of dredgers = 2 dredgers

Daily dredging rate by two dredgers = 6,300m3

Sediment loss rate (S-factor) = 25 kg/m3

Sediment release rate = =

4.4.3.4The sediment release due to grab dredging is
assumed to be continuous and the sediment load is allocated in the whole water
column to represent the sediment loss during the lift motion of the grab.

4.4.3.7It is assumed that the percentage of loss rate
(% of the disturbed sediment becomes suspension) is 20%[1].Based on the sediment analysis for this
Project, the dry density of the sediment is about 1,105 kg/m3.The sediment release rate for jetting is
therefore:

Sediment release rate (kg/s) = =

4.4.3.8Release of sediment is concentrated at the
bottom layer of the water column for jetting and is assumed as a continuous
moving source at a speed of 150 m/hr along the offshore transmission power
cable sections and at the foundation site.A 16 working hours per day with 6 working days per week is assumed for
the jetting operation in this area.

4.4.3.9During the suction caisson installation, water
inside the suction caisson would be pumped out and discharged into the
surrounding water.The total amount
of water to be pumped out of each foundation is not expected to exceed 8,500 m3.The pumping rate would not exceed 300 m3
/ hour per pump, or 1,200 m3 / hour per foundation.

4.4.3.10It is assumed that at the beginning of the
operation the water pumped from the suction caissons would be free of suspended
solids as the upper water layer within the foundation would be extracted.As the pumping progresses it may be
expected that the lowest water layer within the foundation would contain a
certain amount of suspended solids from the seawater / sediment interface.

4.4.3.11To
verify these assumptions a field trial was conducted in May 2008 to measure
turbidity and / or SS concentrations at the discharge location and a various
points downstream from the discharge location. Field measurements revealed that
the increase in SS above ambient levels was negligible throughout the trial
installation, with no sediment plume was observed using underwater video or
visible at the water surface.

4.4.3.12To take
a conservative approach for estimating the sediment release rate used in this water quality impact assessment, it is assumed that
the water pumped out from the suction caissons contains an average of 15%
sediment, which has been verified to
be much higher than the result of the field water quality monitoring of a field
trial presented in Section 4.7.2.The dry density of the sediment, as
determined through fieldwork, is 1,105 kg/m3.The sediment concentration in the
water pumped out from the suction caissons is thus 165.8 kg/m3.

4.4.3.13With reference to the Liquefied Natural Gas
Receiving Terminal and Associated Facilities EIA, 80% of the sediment would
fall from the water column to the seabed within a 70 m radius.The percentage of the disturbed sediment
in suspension is assumed to be 20%.The sediment release rate for each foundation site has therefore been
calculated as:

=

4.4.3.14The suction pumps are installed at the top of
the suction caissons.Discharge of
the water would be conservatively assumed to be highest at 10 m above the seabed
as the whole suction caisson will penetrate into the seabed in time.It is also conservatively assumed that
the duration of the discharge is 8 hours for each foundation.

4.4.3.15As shown in Figure 4.1, the proposed
cable route has been divided into three sections in order to derive the
worst-case scenarios.Section 1
represents the transmission cable section in JunkBay
that requires anchor protection to be put in place within a dredged
trench.Two sediment release points
(P1 and P2) have been nominated within this section for different scenarios.

4.4.3.16No more than two grab dredgers would be deployed
and operate at the same time with a minimum separation of 100 m at each
proposed sediment release point.Sediment release point P1 is located near the Seawater Intakes for WSD
Pumping Station at Tseung Kwan O and the coral communities at Chiu Keng
Wan.Sediment release point P2 is
selected, so that it is located near the Coral Communities at Fat Tong Chau
West.

4.4.3.17Sections 2 and 3 represent the remaining
offshore transmission power cable sections.Installation of the cables will be by
jetting only.Sediment
release points P3 and P4 are the sources representing the movement of the
jetting machine within Section 2 and Section 3 respectively.Only one jetting machine would be
deployed for cable laying.Therefore, jetting can only take place at one location in the entire
Project area at any one time.The
jetting operation for this Project takes only one pass per cable installation
to fluidize the sediment and lay the cable.

4.4.3.18The distance of each of the two sections is
approximately 11 km.Considering
the maximum jetting speed of the jetting machine of 150 m/hr, the jetting
operation can be expected to take 6 - 9 days depending on the actual length of
the working day.As the period
required to complete a single pass is less than the model simulation period of
15 days, it is conservatively assumed that the jetting machine continuously
moves along the section throughout the entire simulation period.This conservative approach covers
different tidal stages during the release of sediment from the jetting machine.

4.4.3.19At the wind farm foundation site, there would be
a maximum of three foundations installed concurrently.Three sediment release points (P5, P6
and P7) which are the closest to the dredging site in Junk Bay and jetting
operation of the transmission power cable sections are allocated on the south-eastern
boundary of the foundation site to take into account the worst situation of
cumulative impact from the construction activities of the Project at the
western side of the Study Area.These sediment release point locations are also near the coral communities
at Tuen Chau Tsai East and at One Foot Rock to represent the worst situation.

4.4.3.20In the case where foundation installations are
carried out near the Victor Rock, which is one of the identified WSRs, three
sediment release points (P8, P9 and P10) allocated at the north-eastern
boundary of the foundation site in the closest proximity to this WSR are
selected.Jetting for the array
cable laying is also considered to be conducted adjacent to these points to
represent the worst situation that may adversely affect the coral communities
at Victor Rock.A moving source at
sediment release point (P11) is used to represent the operation of the jetting
machine.

4.4.3.21There are in total five worst-case scenarios for
water quality impact assessment developed from a combination of different
sediment release points that represent different construction activities for
the entire project area.Table 4.4 presents all the worst-case scenarios.

4.4.3.22Scenario 1 is to simulate the impacts due to dredging
at the nearest point to the seawater intakes for the WSD pumping station at
Tseung Kwan O and coral communities at Chiu Keng Wan in JunkBay.Dredging takes place at sediment release
point P1 in Section 1.In order to
take into account the potential impacts from the other activities of the same
Project, jetting in Section 2 at source P3 and installation of three
foundations at the foundation site (as represented by sediment release points
P5 to P7) are also included to form this worst-case scenario.

4.4.3.23Scenario 2 is similar to Scenario 1 but jetting
takes place in Section 3 of the transmission power cable section as represented
by a moving source P4 for.Dredging
also occurs at P1 in Section 1 of the transmission power cable section and water
pumping operation takes place at P5 to P7 at the foundation site.

4.4.3.24Scenario 3 is to simulate the situation where
dredging takes place nearest to the coral communities at Fat Tong Chau West in JunkBay.Release of sediment due to dredging
operation is at P2 in Section 1 and concurrent Jetting is assumed in Section 2
at the moving source P3.Three
sediment release points (P5, P6 and P7) representing the water pumping
operation are located at the foundation site.

4.4.3.25Scenario 4 is similar to Scenario 3 but jetting
takes place in Section 3 of the transmission power cable section as represented
by a moving source P4.Dredging is
also assumed to carry out at P2 in Section 1, which is located nearest to the
coral communities at Fat Tong Chau West.Water pumping operation takes place at P5 to P7 within the foundation
site.

4.4.3.26Scenario 5 is to simulate the situation where
jetting and water pumping operation for installation of three foundations are
located nearest to Victor Rock.The
jetting operation is represented by a moving source P11 for release of sediment
and water pumping operation is represented by sediment release points P8, P9
and P10 at the north-eastern boundary of the foundation site.Dredging is assumed to carry out at
sediment release point P2 in section 1 of the transmission power cable section.

4.4.3.27The projects or activities that would be carried
out concurrently with this Project and are located near the Works include
Tseung Kwan O Development and East Tung Lung Chau and East Ninepins mud
disposal area.The assessment of
cumulative impacts takes into account the sediment release from these projects
in the five worst-case scenarios. EIA’s for the Cruise Terminal at Kai Tak
project and the Wan Chai Development Phase II project have suggested that coral
colonies be translocated from their current locations to small sites in JunkBay.
Figure 4.2 shows that these
potential coral translocation sites are approximately 1.6 kilometers away from
the cable corridor. As a result, no adverse impacts are anticipated at the
potential coral translocation sites..

4.4.3.28The reclamation activity of Tsueng Kwan O
Development together with the dredging operation of this Project may further increase
the SS elevation in JunkBay, and is included in
all the worst-case scenarios for cumulative impact assessment.The operation of the East Tung Lung Chau
mud disposal area would not overlap with the disposal activity at East Ninepins
mud disposal area.Sediment release
from the disposal activity at East Tung Lung Chau mud disposal area is only
included in Scenarios S1 and S3 where jetting operation takes place near this
disposal area.Jetting operation as
considered in Scenarios 2 and 4 would be carried out near the East Ninepins mud
disposal area.Therefore, sediment
release from the disposal activity at East Ninepins mud disposal area is
included in Scenarios 2 and 4 for cumulative impact assessment.The disposal activity at East Ninepins,
which is also located near the dredging operation in JunkBay,
is included in Scenario 5 together with the jetting operation at the foundation
site to form the worst-case scenario for cumulative impact assessment.

4.4.3.29The approach of this study is to first examine
the worst-case scenarios without any mitigation measures for reducing sediment
release from jetting, dredging and water pumping operations.Mitigated scenarios are, however, also
included in the assessment to achieve compliance with the WQOs.Therefore, the water quality impacts during the construction stage
of the Project examine both the unmitigated and mitigated scenarios.

4.4.3.30During
the operational stage, the model
runs also include drogue
tracking for oil spill to assess the areas that are potentially affected by any potential oil spill events.

4.4.4.1The sub-structures of the wind turbines that are
submerged in the sea cause friction on tidal flow.The following method is used to account
for the hydrodynamic impact due to the submerged sub-structures.

4.4.4.2As the hydrodynamic model grid size is larger
than the wind turbine (~30m diameter[2]),
it is not practicable to correspondingly refine the model grid size as the
computational time would be significantly increased.The frictional effects due to submerged
bridge piers or vertical structures were modelled and assessed in other EIA
studies[3].A similar approach is therefore adopted
in this study to model and assess frictional effects caused by the
sub-structures of the wind turbines.With this approach, additional quadratic friction terms are added to the
momentum equations to represent the frictional effects of wind turbine columns
on the hydrodynamics.The
mathematical expressions for calculating the loss coefficients for accounting
the frictional effects are given as follows:

(4-1)

Where,

is the density of
water;

, and are sizes of the grid
cell in x, y and z directions;

is the velocity, is the magnitude
of the velocity, U and V are velocity components in x and y directions;

and are the loss coefficients
in x and y directions; and

Fx and Fy are drag
forces induced by the sub-structure of the wind turbine in a grid cell, which
are calculated as:

(4-2)

Where,

n is the number of the turbine columns in the
grid cell

is the drag
coefficient;

D is the diameter of the turbine column;

is the effective
approach velocity;

is the magnitude
of the effective velocity, and are the effective
velocity components in x and y directions;

is the total
cross-section area;

is the effective
cross-section area which is the difference between the total cross-section area
and the area blocked by the turbine columns;
and

is the ratio of
the total cross-section area to the effective cross-section area.

Combining Equations (4-1) and
(4-2), the loss coefficient used in the hydrodynamic model is expressed as:

(4-3)

4.4.4.3It is conservatively assumed that the diameter
of the sub-structure is the same as that of the base footprint width of the
foundation, i.e. 30 m.The
estimated loss coefficient for the sub-structure in the wind farm location is
about 0.2.

4.5.1.1To assess the existing water quality conditions
in the study area covering the MirsBay, Port Shelter,
Eastern Buffer and Junk Bay WCZs, the most recently published monitoring data
collected at the EPD marine water monitoring stations near the proposed wind
farm and transmission power cable route have been reviewed.The data can be used to represent the
baseline water quality conditions at representative water sensitive
receivers.

4.5.1.2The selected EPD marine water monitoring
stations include MM8, MM9 and MM14 in the Mirs Bay WCZ;
PM1, PM4, PM6, PM7, PM8, PM9 and PM11 in the Port Shelter WCZ; EM1, EM2 and EM3 in the Eastern Buffer WCZ; and JM3 and JM4 in the Junk Bay WCZ.A summary of EPD monitoring data
collected in between 2002 and 2006 is presented in Table
4.5 to Table 4.8.

2.Data presented are arithmetic means of the
depth-averaged results except for E. coli and faecal coliforms, which are
annual geometric means.

3.Data in brackets indicate the ranges.

4.5.1.3Between 2002 and 2006, the water quality
conditions in the JunkBay,
MirsBay, Port Shelter and Eastern Buffer
WCZs were satisfactory and had high compliance with WQOs.The water quality at Eastern Buffer and
Junk Bay WCZs had 100% full compliance with the key WQOs.The water quality in these WCZs has
improved when compared to the WQO compliance in 2001 after the implementation
of the Harbour Area Treatment Scheme (HATS) Stage 1.

4.5.1.4As shown in Table 4.5 and Table 4.8, the water
quality conditions of Junk Bay WCZ at Stations JM3 and JM4, and Eastern Buffer
WCZ at Stations EM1, EM2 and EM3 fully complied with the key WQOs including DO,
SS and E. coli.

4.5.1.5According to EPD data from 1986 to 2001, the MirsBay
and Port Shelter WCZs were subjected to effluent discharge from the Sha Tin and
Tai Po Sewage Treatment Works and Red tides and fish kills frequently occurred
in the 1990s.

4.5.1.6After implementation of the Tolo Harbour Action
Plan, the water quality in ToloHarbour has sharply
improved.In 2005, both Mirs Bay
WCZ at Stations MM8, MM9 and MM10 and Port Shelter at Stations PM1, PM4, PM 6,
PM7, PM8, PM9 and PM11 fully complied with the WQOs for DO, SS, chlorophyll-a
and E. coli as shown in Table 4.6 and
Table 4.7.

4.5.2.1Water sensitive receivers (WSRs) located within
the WCZs that could potentially be affected by this Project are listed below
and their locations are shown in Figure 4.3, Figure 4.3a and Figure 4.3b.

4.5.2.2In order to systematically present the findings
of the water quality impact assessment, every key WSR is assigned with a
reference identifier.A list of all
the WSRs that have been agreed with EPD and AFCD is presented in Appendix
4B.The shortest
distances of the marine works to the identified WSRs are also shown in this
Appendix.

4.6.1.2Table 4.9
presents the proposed assessment criteria for SS and DO at the WSRs.The WQO for SS specifies that human
activity or waste discharges shall not raise the ambient SS level by 30% and
shall not affect aquatic communities.Appendix 4C summarises
the allowable SS elevations for different categories of WSRs.The ambient SS level at each of the WSRs
was calculated based on the field data from 2002-2006 collected at the EPD’s
marine water monitoring stations that are the nearest to the WSRs.

4.6.1.3There is no existing legislative standard or
guideline in Hong Kong for individual heavy
metals and micro-organic pollutants (PCBs, PAHs and TBT) in marine waters.In the past EIA studies, various
international standards were adopted as the most applicable assessment
criteria.For the present study,
comparisons were made amongst standards of EU, Japan,
USA, UK, Australia
and Singapore.A conservative selection was carried out
using the lowest limiting values from different international standards as the
assessment criteria.Table 4.10
presents the criteria for the evaluation of impacts due to heavy metals and
organic compounds at WSRs.

Note 1: WQO for SS refer to the Water Quality Objective
for suspended solids for various WCZs stipulated under WPCO.The WQO specifies that human activity
or waste discharges shall not raise the ambient SS level by 30% and shall not
affect aquatic communities. Details of the allowable SS elevations for WSRs
are summarised in Appenxdix 4C.

4.7.1.1Land based construction activities are not
included in this Study.The
construction principle activities that may cause water quality impact during
the construction stages of the Project will be carried out in the sea and are
listed below:

·Dredging
and anchor protection for the installation of transmission power cable in JunkBay
using dredgers;

·Jetting for
the installation of transmission power cable connecting to the section in JunkBay
to the offshore foundation site;

·Jetting for
the installation of collection power cable within the foundation site;

·Installation
of foundation involving pumping out seawater from inside of the suction
caissons to the surrounding ambient water;

·Sewage
generation due to workforce; and

·Accidental
spillage of chemicals.

4.7.1.2Project development requires marine works to
install turbine foundations and cables.To prevent damage from anchors and other potential objects, the
transmission cable within JunkBay of approximately 3 km
long shall be buried at about 5 m below the seabed, and shall be overlain with
rock-amour to a level contiguous with the surrounding seabed.The remaining transmission cable
alignment of approximately 21 km and the array cables within the wind farm
footprint shall be buried at about 3 to 5 meters below seabed. Figure 5.2 shows
that the transmission cable alignment does not pass through any key coral
communities.

4.7.1.3There are two transmission power cables for
power transmission from the offshore transformer station to a substation
facility on land. The location of the transformer station is illustrated in Figure 4.1. These two
transmission cables will be buried approximately 50m apart;
whilst in JunkBay
where cables will be buried in the same trench and covered with rock amour
protection. Each jetting operation is a distinct event with impacts of a
similar magnitude to the scenarios modeled. It is identified in Section 2.8.6
that the jetting of the cables will take approximately 2 months to complete for
each run.Therefore any impacts of
the operations adjacent to the site will be significantly separated by many
tidal cycles and cumulative impacts of suspended sediments are not anticipated

4.7.1.4Two closed grab dredgers will be deployed for
removing marine sediment along the transmission power cable section in JunkBay,
and the two cables shall be laid at the bottom of the trench side by side.The fine content of the rock materials
is generally low and the rock materials used for backfilling would be
inert.As such, the water quality
impact associated with rock fill will be low.

4.7.1.5The major concern is sediment release from
dredging activity that may cause elevated SS levels in ambient waters leading
to reduced sunlight penetration, mobilization of contaminants, and possible
direct or induced effects on water sensitive receivers.

4.7.1.6The remaining transmission power cable and the
collection power cables within the wind farm site will be installed by jetting which
uses a strong water jet to fluidise the seabed generating a mixture of water
and sediment close to the seabed.Dispersion of the sediment plumes may affect water sensitive receivers
located on or near the seabed such as coral communities.Since the sediment plumes are generated
at the bottom layer of the water column where the flow velocity is low due to
the bottom friction from the seabed, the SS would normally settle back onto the
seabed quickly.

4.7.1.7Installation of wind turbine foundation
initially makes use of gravity where the suction caissons are driven down into
the seabed by the weight of the foundation structure and suction.When gravity is balanced out by the
frictional force, seawater inside the suction caissons will be mechanically
pumped out to reduce the water pressure inside the suction caissons, thereby
generating a net downward pressure to ease the foundation further into the
seabed with minimal disturbance.

4.7.1.8The seawater pumped from the suction caisson
foundation may contain a small amount of sediment. It is anticipated that at
the beginning of the pumping process, the SS content in the pumped out water
should be very low and would gradually increase when the suction caissons are
almost completely penetrated into the seabed.Similar to the dredging and jetting
activities, dispersion of the SS may impact nearby water sensitive
receivers.Sediment dispersion
modelling has been conducted to predict and assess the potential impact due to
the dredging and jetting for cable installation and pumping of seawater from
the suction caisson foundations.

4.7.1.9The proposed wind farm also comprises of a
transformer station. Its foundation will also be installed using the same type
of suction caisson foundation technique. Thus, the potential water quality impacts
as a result from the foundation installation works will be the same as those
predicted for wind turbine foundation.

4.7.1.10The construction activities may involve the use
of chemicals such as paint, chemical solvents, mineral oils and fuel oil.Accidental spillage of these chemicals
into the seawater could be harmful to the aquatic life.The risk of accidental spillage of
chemicals can be reduced by implementation of good management practice.Practicable and effective
EM&A requirements are presented in the EM&A Manual of this Study.Considering that the amount of
chemicals to be used in the construction activities would be small, the
potential impact of water pollution due to accidental spillage of chemicals is
low.

4.7.2.1The proposed foundation works represent a new
construction technique in the marine environment of the HKSAR.Accordingly, turbidity and suspended
solids data were obtained from the field measurements and sampling conducted in
May 2008 during the site trial for suction caisson installation to verify that
installation would not result in adverse water quality impacts and that the
assumptions used in the impact assessment were suitably conservative, or at
least would not lead to an under-representation of impacts upon the water
sensitive receivers.

4.7.2.2The physical parameters adopted for the site
trial were as follows:

·Caisson
dimension = 3.5 m (diameter) x 12 m (height);

·Pumping
rate ≤ 200 m3
/ hour; and

·Installation
duration = 75 minutes (between 15:45 and 17:00).

4.7.2.3Three sampling distances were selected to
provide water quality data:

·S1
– the immediate vicinity of the source;

·S2
– 70m downstream from the source where 80% reduction in the suspended sediment
level was assumed; and

·S3
– 120m downstream from the source or 50m from S2.

4.7.2.4Figure
4.4 illustrates the schematic arrangement of these sampling stations.

4.7.2.5Additionally, two sampling depths, 10m and 5m
above seabed were adopted for Stations S2 and S3 to represent the upper
boundary and the centre of the trajectory of sediment discharge from the
foundation as predicted by the mathematical model. Grab samples and in-situ
measurements were taken sequentially from the locations every 15 minutes.

4.7.2.6Turbidity and suspended solid baselines were
established through reference sample collection conducted prior to the
installation works.

4.7.2.7Figure
4.5 presents the results of in-situ turbidity data measured at S1,
S2 and S3 during and after installation.The result indicates that the overall turbidity at all stations was low
and mostly below baseline level.

4.7.2.8Although a short-lasting spike in turbidity was
recorded at S1 at the beginning of the installation, such increase decayed
rapidly and was returned back to below the baseline level within 10 minutes as
the installation progressed.The turbidity levels recorded at S1 during the remaining course of
installation was steadily low, which reflects no apparent increase in suspended
solids in ambient water resulting from discharge of water pumped out from the
suction can contained very low level of suspended solids.

4.7.2.9Moreover, it is noted that this sudden increase
in turbidity at S1 was not detected at either of the downstream stations S2 or
S3.The turbidity data recorded at
these two stations during and after the installation was consistently steady
and below the baseline levels.

4.7.2.10Likewise, the suspended solid levels recorded at
S2 and S3 were steadily low at both of the sampling depths of these stations,
as illustrated in Figure 4.6, which are consistent
with the turbidity results.The
measured results are either at or below baseline levels indicate no significant
increase in suspended solid levels resulting from the installation.

4.7.2.11The field monitoring demonstrates that the
results predicted by water quality modeling is significantly more conservative
than the actual field installation, thus no or insignificant water quality
impact arising from the installation of caisson foundation is anticipated.

4.7.3.1The potential water quality impacts in the
construction stage of the Project are mainly due to the sediment dispersion and release
of pollutants, which are
originally adhered on the sediment, from the foundation installation and
cabling works.Disturbance to the marine sediment in the
seabed causes suspension of the sediment in the water column.

4.7.3.2The
foundation installation and cabling works, however, would not introduce
additional sources of pollutant into the water column. Suspended solids
(SS) and dissolved oxygen (DO) are the key water quality parameters that need to be assessed and compared
against relevant criteria.The
Delft3D fine grid model was used to model the proposed worst-case scenarios and
to simulate the sediment dispersion in the water environment. The following
presents the predicted results of SS and DO without implementation of any
mitigation measures:

Scenario 1

4.7.3.3Appendix
4D includes the predicted increases in SS at all the WSRs for
Scenario 1.The majority of the
WSRs did not show detectable increases in SS, i.e. increase in SS is zero.In order to show clearly which WSRs
would be affected by the construction activities of the Project, the WSRs with
detectable increases in SS, i.e. > 0.01 mg/L, in either the dry season or
the wet season are presented in Table 4.11.The other WSRs with no detectable
increases in SS are not presented in the table but can still be found in Appendix
4D.

4.7.3.4The coral communities at Junk Bay (CC26), Junk
Island (CC27), Fat Tong Chau West (CC11) and seawater intake at Tseung Kwan O
(SW13) in JunkBay would be affected by the dredging
and jetting operations.The
increases in maximum SS in the wet season at the coral communities at JunkBay
(3.03 mg/L), JunkIsland (4.79 mg/L) and
Fat Tong Chau West (2.97 mg/L) were higher than the allowable limit (2.03
mg/L).The time
series plots for increases in SS at CC11, CC26 and CC27 presented in Figure 4.7
show the high peaks of SS above the allowable SS elevations for this
scenario.Mitigation measures
should be implemented to reduce the SS elevations at these WSRs to a level
below the allowable limit.

4.7.3.5The increases in mean SS during the dry season
(0.00 – 0.03 mg/L) and the wet season (0.00 – 0.36) at these WSRs were below
the allowable limits.The average
mean values of the increases in SS were also well below the allowable
limits.

4.7.3.6It is likely that both the jetting operation
(represented by sediment release point P3) and the dredging operation in JunkBay
(represented by sediment release point P1) contribute to the high peak SS
levels at these locations.The
combined effects would be reduced when the jetting machine moves away from JunkBay.Hence, elevation of SS would be
reduced.

4.7.3.7There would be slight increases in SS levels at
the site with amphioxus occurrence (AO8), which is located to the southeast of
Tung Lung Chau.The increases were
small, i.e. increases in mean SS were 0.03 mg/L in the dry season and 0.02 in
the wet season.The increases in maximum
SS in the dry season (2.18 mg/L) and in the wet season (1.25 mg/L) were below
the allowable limits.

Remarks: 1.Values
of the increases in SS are depth-averaged SS concentrations. 2.The figure in boldrepresents that the
predicted SS concentration is higher than the allowable SS elevation.

Scenario 2

4.7.3.8The predicted SS elevations at all the WSRs are
included in Appendix
4D.There were no SS
elevations at most of the WSRs.Table 4.12 shows the predicted SS elevations at the
WSRs with detectable increases in SS for Scenario 2.The WSRs with detectable increases in SS
were the coral communities at Junk Bay (CC26), the site with amphioxus
occurrence (AO9) and sighting points of marine mammal (MM8 and MM11).The maximum increase in SS (3.03 mg/L)
at coral communities at Junk Bay (CC26) in the wet season was higher than the
allowable limit (2.03 mg/L).Figure 4.8
shows the time series plot of the
predicted SS with exceedance in allowable limit during the occurrence of high
peaks of SS.

4.7.3.9All the seasonal and average mean SS increases
were however below the allowable limits.The transient high peaks of SS at CC26 would be mainly due to dredging.

Remarks:1Values of the increases in SS are depth-averaged SS concentrations. 2.The figure in boldrepresents that the
predicted SS concentration is higher than the allowable SS elevation.

Scenario 3

4.7.3.10The predicted SS elevations at all the WSRs are
included in Appendix
4D.There were no SS
elevations at most of the WSRs.The
predicted SS elevations at the WSRs with detectable increases in SS for
Scenario 3 are presented in Table 4.13.Increases in SS were only detected at
coral communities at Fat Tong Chau West (CC11) and at the site with amphioxus
occurrence (AO8).

4.7.3.11There was no exceedance of the increases in
seasonal and average mean SS of the dry and wet seasons.However, the increases in maximum SS in
the dry season (5.44 mg/L) and in the wet season (10.26 mg/L) at CC11 exceeded
the corresponding allowable limits (2.24 mg/L for the dry season and 2.03 mg/L
for the wet season).The time series plots for increases in SS at CC11 during
both the dry and wet seasons are shown in Figure 4.9.Exceedances during the high peaks of SS
are clearly shown in the time series
plots.It is worth noting that no
mitigation measures are considered in this scenario.

Remarks:1. Values of the increases in SS are
depth-averaged SS concentrations. 2. The figure in bold represents that the predicted SS concentration is
higher than the allowable SS elevation.

Scenario 4

4.7.3.12The predicted SS elevations with detectable
increases in SS for Scenario 4 are presented in Table
4.14.A complete list of the
predicted SS elevations at all the WSRs are included in Appendix
4D.Increases in SS were
recorded at coral communities at Fat Tong Chau West (CC11), the site with
amphioxus occurrence (AO9), and sighting points of marine mammal (MM8 and
MM11).

4.7.3.13Based on the model predictions for this
unmitigated scenario, there was no exceedance of the increases in seasonal mean
and average mean SS of the dry and wet seasons at these WSRs.However, the increases in maximum SS at
CC11 in the dry season (4.93 mg/L) and in the wet season (7.29 mg/L) exceeded
the corresponding allowable limits.

4.7.3.14The time
series plots for increases in SS at CC11 during the dry and wet seasons in Figure 4.10
show the SS exceedances at different time
intervals.The exceedances would be
related to the dredging operation in JunkBay.

Remarks:1.
Values of the increases in SS are depth-averaged SS concentrations. 2. The figure in boldrepresents that the predicted SS concentration is higher than the
allowable SS elevation.

Scenario 5

4.7.3.15The predicted SS elevations at all the WSRs are
included in Appendix
4D.Except at CC11,
there were no SS elevations at the other WSRs.Table 4.15
presents the increases in SS at CC11 for Scenario 5.The jetting operation was allocated at the wind farm.The only detectable increases in SS were
at coral communities at Fat Tong Chau West (CC11).The increases in maximum SS in the dry
season (4.93 mg/L) and in the wet season (7.29 mg/L) were higher than the
corresponding allowable limits (2.24 mg/L in the dry season and 2.03 in
the wet season).The time series plot for the increases in SS at CC11
during the dry and wet seasons are also shown in Figure 4.9.The results were the same as those of
the Scenario 4.WQO exceedances for
SS frequently occurred over the simulation period.Since the location of CC11 is far away
from the foundation site, the model predicted almost no influence from the
jetting and water pumping activities at the foundation site.